The economic and environmental factors relating to upgrading of petroleum residual oils and other heavy hydrocarbon feedstocks have encouraged efforts to provide improved processing technology, as exemplified by the disclosures of various U.S. patents which include U.S. Pat. Nos. 3,696,027; 3,730,879; 3,775,303; 3,876,530; 3,882,049; 3,897,329; 3,905,893; 3,901,792; 3,964,995; 3,985,643; 4,016,067; 4,263,129; and the like.
Visbreaking is a mild cracking operation employed to reduce the viscosity of heavy residua. The heavy residua are sometimes blended with valuable lighter oil, or cutter stocks, to produce fuel oils of acceptable viscosity. By use of visbreakers, the viscosity of the heavy residua is reduced so as to reduce the requirement of the cutter stock. The ultimate objective of the visbreaking operation is to completely eliminate the need for cutter stocks.
Sometimes visbreakers are also used to generate more gas oils for catalytic cracking and naphtha for reforming to increase the gasoline yield in the overall refining operation. To achieve these goals, the visbreaker has to be operated at high enough severity to generate sufficient quantities of lighter products.
If visbreaking of heavy hydrocarbon oil is conducted in the presence of an acidic catalyst such as HZSM-5, there is some disadvantage as to the quantity of C.sub.1 -C.sub.3 hydrocarbons produced, and the catalyst tends to experience deactivation in the presence of metals and sulfur.
If visbreaking of heavy hydrocarbon oil is conducted in the presence of a low acidity catalyst such as NaZSM-5, the said catalyst exhibits excellent stability and ageing properties as described in U.S. Pat. No. 4,263,129. However, the residual 1000.degree. F.+ fraction of the visbroken effluent is characterized by a high viscosity which necessitates blending with cutter stock, and concomitantly this represents a decrease in net distillate production.
Accordingly, it is an object of this invention to provide an improved process for upgrading heavy hydrocarbon oils for use as liquid fuels and as demetalized and desulfurized feedstocks for refinery cracking operations.
It is another object of this invention to provide a catalytic hydrovisbreaking process in which the catalyst exhibits resistance to metals, nitrogen and sulfur and is characterized by long term on-stream stability.
It is a further object of this invention to provide a catalytic process for hydrovisbreaking heavy hydrocarbon oils to increase the yield of distillate products and at the same time produce a residual 1000.degree. F.+ fraction which has a relatively low viscosity that requires little or no cutter stock to meet heavy fuel oil specifications.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.